Fix HttpEntity support with Kotlin Serialization

This commit adds HttpEntity type unwrapping logic to
KotlinRequestBodyAdvice and KotlinResponseBodyAdvice.

Closes gh-35281

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/KotlinRequestBodyAdvice.java

@@ -28,6 +28,7 @@ import kotlin.reflect.jvm.ReflectJvmMapping;
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.http.HttpEntity;
 import org.springframework.http.converter.AbstractKotlinSerializationHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
 import org.springframework.http.converter.SmartHttpMessageConverter;
@@ -61,6 +62,10 @@ public class KotlinRequestBodyAdvice extends RequestBodyAdviceAdapter {
 		for (KParameter p : Objects.requireNonNull(function).getParameters()) {
 			if (KParameter.Kind.VALUE.equals(p.getKind())) {
 				if (index == i++) {
+					if (HttpEntity.class.isAssignableFrom(parameter.getParameterType())) {
+						return Collections.singletonMap(KType.class.getName(),
+								Objects.requireNonNull(p.getType().getArguments().get(0).getType()));
+					}
 					return Collections.singletonMap(KType.class.getName(), p.getType());
 				}
 			}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/KotlinResponseBodyAdvice.java

@@ -26,6 +26,7 @@ import kotlin.reflect.jvm.ReflectJvmMapping;
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.core.MethodParameter;
+import org.springframework.http.HttpEntity;
 import org.springframework.http.MediaType;
 import org.springframework.http.converter.AbstractKotlinSerializationHttpMessageConverter;
 import org.springframework.http.converter.HttpMessageConverter;
@@ -61,6 +62,9 @@ public class KotlinResponseBodyAdvice implements ResponseBodyAdvice<Object> {
 
 		KFunction<?> function = ReflectJvmMapping.getKotlinFunction(Objects.requireNonNull(returnType.getMethod()));
 		KType type = Objects.requireNonNull(function).getReturnType();
+		if (HttpEntity.class.isAssignableFrom(returnType.getParameterType())) {
+			return Collections.singletonMap(KType.class.getName(), Objects.requireNonNull(type.getArguments().get(0).getType()));
+		}
 		return Collections.singletonMap(KType.class.getName(), type);
 	}
 

spring-webmvc/src/test/kotlin/org/springframework/web/servlet/mvc/method/annotation/RequestResponseBodyMethodProcessorKotlinTests.kt

@@ -20,6 +20,8 @@ import kotlinx.serialization.Serializable
 import org.assertj.core.api.Assertions
 import org.junit.jupiter.api.Test
 import org.springframework.core.MethodParameter
+import org.springframework.http.RequestEntity
+import org.springframework.http.ResponseEntity
 import org.springframework.http.converter.StringHttpMessageConverter
 import org.springframework.http.converter.json.KotlinSerializationJsonHttpMessageConverter
 import org.springframework.validation.beanvalidation.LocalValidatorFactoryBean
@@ -68,6 +70,22 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 			.contains("\"value\":\"foo\"")
 	}
 
+	@Test
+	fun writeEntityWithKotlinSerializationJsonMessageConverter() {
+		val method = SampleController::writeMessageEntity::javaMethod.get()!!
+		val handlerMethod = HandlerMethod(SampleController(), method)
+		val methodReturnType = handlerMethod.returnType
+
+		val converters = listOf(KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, listOf(KotlinResponseBodyAdvice()))
+
+		val returnValue: Any? = SampleController().writeMessageEntity().body
+		processor.handleReturnValue(returnValue, methodReturnType, this.container, this.request)
+
+		Assertions.assertThat(this.servletResponse.contentAsString)
+			.contains("\"value\":\"foo\"")
+	}
+
 	@Test
 	fun writeGenericTypeWithKotlinSerializationJsonMessageConverter() {
 		val method = SampleController::writeMessages::javaMethod.get()!!
@@ -118,6 +136,24 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 		Assertions.assertThat(result).isEqualTo(Message("foo"))
 	}
 
+	@Test
+	@Suppress("UNCHECKED_CAST")
+	fun readEntityWithKotlinSerializationJsonMessageConverter() {
+		val content = "{\"value\" : \"foo\"}"
+		this.servletRequest.setContent(content.toByteArray(StandardCharsets.UTF_8))
+		this.servletRequest.setContentType("application/json")
+
+		val converters = listOf(StringHttpMessageConverter(), KotlinSerializationJsonHttpMessageConverter())
+		val processor = RequestResponseBodyMethodProcessor(converters, null, listOf(KotlinRequestBodyAdvice()))
+
+		val method = SampleController::readMessageEntity::javaMethod.get()!!
+		val methodParameter = MethodParameter(method, 0)
+
+		val result = processor.resolveArgument(methodParameter, container, request, factory) as Message
+
+		Assertions.assertThat(result).isEqualTo(Message("foo"))
+	}
+
 	@Suppress("UNCHECKED_CAST")
 	@Test
 	fun readGenericTypeWithKotlinSerializationJsonMessageConverter() {
@@ -161,6 +197,10 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 		@ResponseBody
 		fun writeMessage() = Message("foo")
 
+		@RequestMapping
+		@ResponseBody
+		fun writeMessageEntity() = ResponseEntity.ok(Message("foo"))
+
 		@RequestMapping
 		@ResponseBody
 		fun writeMessages() = listOf(Message("foo"), Message("bar"))
@@ -169,6 +209,10 @@ class RequestResponseBodyMethodProcessorKotlinTests {
 		@ResponseBody
 		fun readMessage(message: Message) = message.value
 
+		@RequestMapping
+		@ResponseBody
+		fun readMessageEntity(entity: RequestEntity<Message>) = entity.body!!.value
+
 		@RequestMapping
 		@ResponseBody
 		fun readMessages(messages: List<Message>) = messages.map { it.value }.reduce { acc, string -> "$acc $string" }